The structure of an aircraft lifting surface usually comprises a torsion box.
For example, an aircraft tail plane (horizontal or vertical) is usually structured by a leading edge, a torsion box and a trailing edge with control surfaces (flaps, elevators, rudders, etc.).
The torsion box is the primary structure responsible for supporting all loads involved (aerodynamic, fuel, dynamics, etc.) and comprises several structural elements.
Composite materials with an organic matrix and continuous fibers, especially CFRP (Carbon Fiber Reinforced Plastic), are nowadays widely used in the aeronautical industry in a great variety of structural elements. Specifically, all the elements which make up the torsion boxes of aircraft tail planes and other lifting surfaces can be manufactured using CFRP.
The design of composite torsion boxes requires combining two perspectives of different nature: that of structural design and that of manufacture.
The traditional approach is the design of the torsion box defining the structural elements that form it (skins, spars, stringers, ribs), the separate manufacture of these elements and their subsequent join in the assembly plant following schemes similar to those used in the aeronautics industry when only metallic materials were used.
The manufacture can be done using prepreg technology. In a first step, a flat lay-up of composite prepreg plies for each element is prepared. Then a laminated preform of the element with the required shape is obtained by means of a classical hot-forming process, being in some cases substituted by a press-forming process due to high curvatures. After getting the required shape, the laminated preform is cured in a male or female tooling depending on the tolerances required and the overall manufacturing cost. In the case of certain elements comprising sub-components cured separately, such as a rib and a vertical stiffener of it, a second curing cycle is needed for co-bonding said sub-components. Finally, after all the curing cycles, the element contours are trimmed getting the final geometry, and then the element is inspected by an ultrasonic system to assure its quality. The cost of a torsion box manufactured with said method is high because said steps shall be carried out independently for each structural element. Additionally, the cost related to the assembly of the torsion box is also high due to the long length and high complexity of the tasks required to install and to fit all structural elements together. This approach is being followed for manufacturing multi-rib torsion boxes such as that of the horizontal tail plane (HTP) shown in FIGS. 1a and 1b. 
The HTP is structured by leading edges 11, torsion boxes 13 and trailing edges 15 with control surfaces (flaps, elevators, rudders, etc.). The structural elements of torsion boxes 13 are upper and lower skins 21, 23 stiffened by longitudinal stringers, a front spar 18, a rear spar 20 and transverse ribs 16 attached to the front and rear spars 18, 20 and to the upper and lower skins 21, 23 in order to keep the torsion box shape and reinforce the load introductions areas linked to the HTP structural arrangement in the aircraft and to the actuators for handling the HTP control surfaces.
An alternative approach is to manufacture the whole or a part of a torsion box in an integrated manner for obtaining a monolithic ensemble comprising all or part of the structural elements of the torsion box. In this respect one example is described in WO 2008/132251 for a multi-spar torsion box.
Since analytical tools to obtain an optimal design of a torsion box of an aircraft tail plane made of composite materials taking into account all the variables involved and especially those related to their manufacture are not available at present, the aeronautics industry is constantly demanding new torsion box proposals and new manufacturing methods that improve efficiency and/or costs of known torsion boxes.
The present invention is directed to the attention of that demand.